1. Field of the Invention
The present invention relates to an MIMO (multiple input multiple output) transmission technology for transmitting a plurality of streams by way of a plurality of antennas.
2. Description of the Related Art
In a wireless telecommunication system, mobile terminals (i.e., terminals) are enabled to continue a telecommunication if a received power, et cetera, is reduced by selectively changing a connected base station over to an optimal one based on a received power, a reception quality, et cetera, from surrounding base stations (e.g., refer to patent documents 1, 2, 3 and 4; and a non-patent document 1). An example of a wireless telecommunication system is the recently propagated W-CDMA (Wideband Code Division Multiple Access) system.
FIG. 1 exemplifies a configuration of a telecommunication system adopting the W-CDMA system. The terminal (i.e., a mobile station, MS) receives signals from a plurality of base transmission stations (BTS) BTSa, BTSb, BTSc through BTSn. The mobile station selects a base transmission station with the largest received power as the optimal base transmission station for the telecommunication from among the plurality thereof to connect to a network via the selected base transmission station. If a received power from a nearby base transmission station (i.e., adjacent base transmission station) becomes larger than that from the connected base transmission station as a result of the mobile station moving for example, a handover, i.e., changing the connected base transmission station to the nearby one, is carried out for a continued telecommunication. A handover is also carried out according to the telecommunication environment in the case of transmitting and receiving a plurality of data streams simultaneously between a base transmission station and mobile station by utilizing the MIMO technique. The following description is of a handover processing utilizing the MIMO technique in a wireless telecommunication system adopting the HSDPA (high speed downlink packet access) as one standard specification of W-CDMA.
FIG. 2 shows an overview of a handover processing according to the conventional technique; illustrating a handover processing in a downlink transmission under the HSDPA system 100, that is, in a data transmission from the base transmission stations to a mobile station. In the example of FIG. 2, a 2 by 2 antenna configuration is structured between a base transmission station and a mobile station.
Prior to a handover, a mobile station 102 receives a data stream via a base transmission station 103A as shown by FIG. 2 (a). As the received power from a base transmission station 103B becomes larger than that of the base transmission station 103A at the mobile station 102, a handover processing changes the base transmission station 103A to the base transmission station 103B with a larger received power, as the base transmission station for transmitting a data stream over, as shown by FIG. 2 (b). In this event, the two data streams transmitted from the base transmission station 103A are simultaneously changed over. The handover processing results in the mobile station 102 receiving two data streams via the base transmission station 103B.
FIGS. 3, 4A and 4B exemplify respective configurations of a receiving apparatus and a transmission apparatus according to the conventional technique. Here, the mobile station 102 is a receiving apparatus, while the base transmission station 103 is the transmission apparatus. And in the examples of FIGS. 3, 4A and 4B, the mobile station 102 and the base transmission station 103 comprise, respectively, three antennas, for transmission and reception, thereby configured to transmit and receive three data streams simultaneously. Incidentally, antennas (i.e., Tx1 through Tx3; and Rx1 through Rx3), utilized for transmitting the data stream from the base transmission station 103 to the mobile station 102, are delineated separately from antennas (i.e., Tx0 and Rx0) for transmitting a signal from the mobile station 102 to the base transmission station 103, for ease of viewing of FIGS. 3, 4A and 4B. But each antenna is used both for transmitting and receiving signals in the actual structure. That is, the antenna Tx0 comprised by the mobile station 102 is achieved by a discretionary one or a plurality of the antennas Rx1 through Rx3, and likewise the antenna Rx0 comprised by the base transmission station 103 is achieved by a discretionary one or a plurality of the antennas Tx1 through Tx3. Note that this also applies to those drawings herein where the transmission and reception antennas are separately delineated in the following descriptions.
The conventional mobile station 102 as shown by FIG. 3 comprises three antennas Rx1, Rx2 and Rx3, and the corresponding receiver units 111A, 111B and 111C. Received power measurement units 112A, 112B and 112C measure the received power from each base transmission station 103 at the respective receiver units 111A, 111B and 111C. A handover judgment unit 113 calculates the total received power, i.e., a sum of the received power at each base transmission station based on the measurement result at each received power measurement unit 112 and judges whether a handover is to be carried out or not.
In the conventional base transmission station 103 as shown by FIG. 4A, the antenna Rx0 receives handover control information transmitted from the antenna Tx0 of the mobile station 102. The received information is provided to a handover control signal extraction unit 131 by way of the receiver unit. The handover control signal is transmitted to an upper level control station, that is, a radio network controller (RNC) in the HSDPA system 100.
FIG. 4B shows a configuration, relating to a handover processing, of an RNC as the upper level station of a base transmission station 103. As a handover signal is transmitted from the base transmission station 103 to the RNC, a handover control unit thereof determines a timing for carrying out a handover A handover control unit 132 of the base transmission station 103 carries out a handover by receiving a notification from the RNC.
FIG. 5 shows a process procedure of a handover in the conventional system. The first step is to measure a received power from each base transmission station (S101), compare the total received power from the surrounding base transmission station of the mobile station 102 (S102), judge whether or not a handover is to be carried out based on the comparison result and, if it is judged that a handover is to be carried out, determine which base transmission station is to be handed over to (S103).
FIG. 6 is a flow chart showing a definite measurement process of a received power as a part of the conventional handover processing. In the example shown by FIG. 6, the n-number of base transmission stations exist in the surrounding area of the mobile station, and each terminal station is equipped by the m-number of antennas respectively. Meanwhile, it is defined that a received power of a signal from the base transmission station of a base transmission station number j is Pj and that the power received by the antenna Ax is Prx.
In the measurement process of a received power, initialize the antenna number, the base transmission station number and the received power from each base transmission station (S111), increment the base transmission station number j and the antenna number i, by one, respectively (S112 and S113) and add the received power Prx when receiving a signal from each base transmission station via each antenna Ai and acquire the sum of the received power (S114, S115 and S116). The sum of received powers is acquired for the n-number of base transmission stations, respectively (S117).
In the conventional handover processing, the necessity of a handover is judged based on the sum of the received powers and when the base transmission stations are changed over from the current base transmission station to a target base transmission station for transmitting a data stream, the data stream is transmitted via a different base transmission station pre- and post- the handover, as shown by FIGS. 5 and 6.
FIG. 7 illustrates data transmission pre- and post- a handover according to the conventional technique. The handover is defined as the mobile station 102 moving from under the base transmission station BTS1 to the BTSh. As shown by FIG. 7 (a), the base transmission station BTS1 transmits data prior to a handover and, when the upper level control station, i.e., an RNC, carries out a handover at a prescribed timing, it cuts off the path between the RNC and base transmission station BTS1 and instead establishes a path between the RNC and another base transmission station BTSh. As shown by FIG. 7 (b), the data is then transmitted via the post-handover base transmission station BTSh, leaving data accumulated in the base transmission station BTS1 prior to the handover un-transmitted.
If the accumulated data in the base transmission station BTS1 as shown by FIG. 7 is left un-transmitted, the mobile station 102 is unable to receive all the data. In order to prevent such a loss of data from occurring, a data transfer or a retransmission has been carried out in the conventional technique.
FIGS. 8 through 11 describe the processing for preventing a data loss at a handover occurrence of the conventional system. In FIGS. 8 through 11, definitions for designations are as follows: a current base transmission station (i.e., the station which hands a terminal over): BTSa, a target base transmission station (i.e., the station which a terminal is handed over to): BTSb, the data delivered to the BTSa and BTSb are data A and data B, respectively.
FIG. 8 describes a state of a buffer at each base transmission station prior to a handover. A radio network controller RNC, i.e., the upper level control station of a base transmission station, transmits data to the mobile station via the connected BTSa. The data A is accumulated in the buffer of the BTSa.
FIG. 9 describes a state of the buffer at each base transmission station immediately after carrying out a handover in the case of retransmitting data after the handover. After carrying out a handover, the mobile station 102 connects itself to a network via the BTSb. The buffer of the BTSb accumulates the data B after carrying out a handover. Meanwhile, the data A which has been delivered to the BTSa before carrying out the handover ends up remaining in the buffer thereof.
FIG. 10 describes a data retransmission processing by the upper level control station. As shown by FIG. 9, the data A is the data transmitted to the current base transmission station BTSa prior to the handover and accumulated in the buffer. The RNC, i.e., the upper level control station, receiving a request from a mobile station 102, retransmits the data A accumulated in the BTSa to the BTSb, that is, the target base transmission station, while the data A in the BTSa is discarded. In the BTSb, a signal exchange between the mobile station 102 and RNC is conducted for a retransmission control prior to a retransmission processing of the data A, followed by retransmitting the data A. An exchange of a retransmission control signal and the actual retransmission processing are time consuming.
FIG. 11 describes a state of a buffer at each base transmission station immediately after carrying out a handover in the case of transferring data after the handover. This method transfers, to the BTSb via the RNC, data A accumulated in a buffer of the BTSa, as a result of a handover. Similarly, for such a transfer, an exchange of a control signal and actual transfer processing are time consuming as with the data retransmission processing shown by FIG. 10.
As described by referring to FIGS. 8 through 11, since a plurality of data streams simultaneously transmitted are changed over at the same time in the conventional technique, a data loss occurs in a mobile station 102 unless the data accumulated in the buffer of a current base transmission station is retransmitted or transferred to the mobile station 102. There is a problem of a handover process being time consuming as much as the processing of retransmitting or transferring the data taking time, hence resulting in a decreased transmission speed.
Meanwhile, in a system for wirelessly and simultaneously transmitting mutually independent data streams respectively from a plurality of transmission systems by using the same frequency, as a MIMO related technique, there is a technique for wirelessly transmitting a transmission data stream simultaneously by using the same frequency, respectively, from a plurality of transmission systems, following changing over from a mutually independent plurality of data streams to a plurality of sub-streams, if a received power of a mobile station (or a reception quality) assumes a prescribed threshold value or less. According to such a technique, a MIMO transmission is carried out if a mobile station exists in the neighborhood of a base transmission station with a relatively large received power, while a diversity transmission is performed if a mobile station exists in an area of a relatively low received power. In the neighborhood of the two areas, a changeover between the MIMO transmission and the diversity transmission is conducted.    [Patent document 1] laid-open Japanese patent application publication No. 2004-72624    [Patent document 2] laid-open Japanese patent application publication No. 2003-338781    [Patent document 3] laid-open Japanese patent application publication No. 2004-229088    [Patent document 4] laid-open Japanese patent application publication No. 2005-509565    [Non-patent document 1] 3GPP Specification: 25.211 Rel-5, Version 5.7.0, [online], Aug. 2, 2005, 3GPP, searched on Aug. 3, 2005, Internet <URL: http://www.3gpp.org/ftp/Specs/html-info/25211.htm>